Migrate customers unavailable to shared haml alert

What does this MR do and why?

Updates the alert on the billing page to use the new renderer.

This is part of the effort in &5213

Screenshots or screen recordings

They're different images, I swear.

Before	After

How to set up and validate locally

There's two ways, the easy way and the hard way. The easy way is potentially inaccurate, the hard way is more accurate but takes longer.

Hard way

1. Follow the guide to enable billing on GDK
   1. https://gitlab.com/gitlab-org/customers-gitlab-com#enabling-the-billing-pages-on-a-local-running-gitlab-instance
2. Remove lines 24 and 26 on this file: https://gitlab.com/gitlab-org/gitlab/-/blob/354601-alert-customers-dot/ee/app/controllers/groups/billings_controller.rb#L24
3. Head to the group billing page.

Easier way

1. Go to the controller for a known page
2. paste the following line in the controller `render 'shared/billings/customers_dot_unavailable'
3. Head to that page and see the alert

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #354601 (closed)